Lecture Topic 18 Mechanics of Breathing 2024 PDF

Physiology of the respiratory system Lecture 18: Mechanics of breathing including assessment of lung function Lecture 19: Blood gas transport Lecture 20: Control of breathing Lecture 21: When things go wrong with the respiratory system  Learning objectives of this lecture topic...

Physiology of the respiratory system Lecture 18: Mechanics of breathing including assessment of lung function Lecture 19: Blood gas transport Lecture 20: Control of breathing Lecture 21: When things go wrong with the respiratory system  Learning objectives of this lecture topic Part 1. Mechanics of breathing - Pressure and volume changes Part 2. What we have to overcome? Physical factors affecting pulmonary ventilation Resistance to breathing: Lung compliance – factors affecting it including surface tension and surfactant Airway resistance: What is it What effect does it have on air flow What factors affect it How can we measure it? Part 3. Assessment of lung function Explain (and calculate where necessary) Respiratory Rates, Volumes and Capacities  Lecture topic 18: Mechanics of breathing Part 1. Pressure & Volume changes This Photo by Unknown Author is licensed under CC BY-SA What is respiration? Exchange of gases (oxygen & carbon dioxide) between atmosphere, blood and cells contributes to homeostasis regulates the pH of the internal environment Respiration takes place in three steps: Pulmonary ventilation (breathing) inspiration and expiration of air between atmosphere and lungs (alveoli) External (pulmonary) respiration exchange of gases between alveoli and blood in pulmonary capillaries Blood gains oxygen and loses carbon dioxide Internal (tissue) respiration exchange of gases between blood in systemic capillaries and tissue cells. Blood loses oxygen and gains carbon dioxide. Carbon dioxide is generated from cellular respiration The mechani cs of breathin g, volume and pressure changes Pulmonary ventilation (breathing) Physical movement of air in and out of the alveoli of the lungs Movement IN = INSPIRATION Movement OUT = EXPIRATION Relies on two physical principles Boyles Law Air flows from an area of high pressure to that of a low pressure Boyle’s Law When the temperature of a gas is constant, the pressure of the gas varies inversely with volume volume decreases volume increases pressure pressure increases decreases Air Flow – from area of high pressure to a low one 1. Plunger pulled back 1. Plunger pushed in 2. Volume ↑ 2. Volume ↓ 3. Pressure in syringe ↓ 3. Pressure in syringe ↑ 4. Air moves in 4. Air moves out (Saladin 2004) Atmospheric pressure = 760 760 mm Hg (0) mm Hg (Martini et al, 2004) Inspiration Expiration 759 (-1) 761 (+1) mm Hg mm Hg 1. Lung volume ↑ 1. Lung volume ↓ 2. Intrapulmonary 2. Intrapulmonary pressure ↓ pressure ↑  Intrapleural pressure Refers to the pressure within the pleural cavity Always lower than atmospheric and intrapulmonary pressures Created by elastic recoil of the lungs (Martini et al, 2004) inspiration expiration  Lecture topic Mechanics of breathing Part 2 – Resistance to breathing This Photo by Unknown Author is licensed under CC BY- Resistance to breathing Forces to overcome: (1)Lung (pulmonary) compliance – this is the ease with which the lungs can be expanded [distensibility] a) elasticity of lung tissue – connective tissue structure b) surface tension of alveoli c) mobility of chest wall (2) Airway resistance: major non-elastic source  Elastic recoil of Insp Exp lung opposes 500 inspiration and Lung aids expiration Volume Surface tension (ml) of alveoli 0 -5 Airway resistance Intra- pleural pressure opposes both (cm H20) -8 if no airway resistance 1. (a) Lung compliance - Elasticity of lung tissue V Compliance  (litres/mm Hg) P Measure of elastic recoil A measure of (lung) volume changes resulting from a given change in pressure V V Compliance  (litres/cm H 2 O) P V P P EXAMPLE A patient inhales 500 ml of air inhaled on a spirometer Intrapleural pressure before inspiration is - 5 cm H2O and -8 cm H2O at the end of inspiration. Then using the equation “Compliance= δV/ δ P (δ = change)” = 0.5L/ (-5cm H2O- [-8cm H2O]) = 0.5L/ (-5cm H2O + 8cm H2O) = 0.5L / 3 cm H2O = 0.1667 L/cm H2O 1. (b) Surface tension caused by intermolecular forces between molecules in a liquid Air-fluid interface surface of fluid is under tension like a thin membrane being stretched = like the thin fluid layer between the alveolar cells and the air (gas- liquid boundary)  Laplace’s Law Describes the relationship between Pressure (P), surface tension (T) and the radius (r) of an alveolus (bubble) 2T P r air At equilibrium, the tendency of increased pressure to expand the alveolus balances the tendency of surface tension to collapse it  Surfactant helps keep uniform alveolar size More concentrated in smaller alveoli (per mm s. area) Lower surface tension helps equalise pressure among alveoli of different sizes Easier to inflate smaller alveoli Work needed to expand alveoli with each breath greatly reduced water Exp - T decreases as alveoli get T surfactant smaller - allows alveoli dynamically Insp adjust their rates of inflation and deflation Surface area Neonatal respiratory distress syndrome Lack of surfactant secretion in premature babies (28-32 weeks gestation) Reduced compliance Alveoli collapse on exhalation Difficult to inflate lungs 50% die without rapid treatment 1. (c) Mobility of thoracic cage  2. Airway resistance Major ‘non-elastic’ source of resistance to gas flow = friction ↑resistance ↓ gas flow r Resistance mainly determined by radius Factors affecting airway resistance 1. Lung volume Bronchi dilate as lung expands 2. Bronchial smooth muscle Parasympathetic nerves bronchoconstriction Sympathetic nerves & adrenaline bronchodilation Stimuli causing reflex bronchoconstriction: smoke, dust, irritants histamine (e.g. allergic response)  Measuring airway resistance: Forced Vital Capacity (FVC) and Forced Expiratory Volume in 1 second (FEV1) 6 5 4 3 FVC – forceably breathing out vital FEV1 FVC 2 - 4.8l - 5.8l capacity 1 FEV1/FVC x 100 Usually little difference with = 83% 0 VC litres [= maximum amount of air that 0 1 2 3 4 seconds can be expired after a maximum inspiratory effort] FEV1 - the volume of air expired in 1 seconds Used to assess changes in resistance to airflow, for e.g. in asthma patients FEV1 most frequently used and expressed as a % of  Mechanics of breathing Part 3: Assessment of lung function This Photo by Unknown Author is licensed under CC BY-  Breath sounds presence of mucous/fluid absence of breath sounds: collapsed lung? Pulmonary function tests Peak flow meter: measures the speed at which you are able to breathe air out used by chronic asthmatics on a regular basis Lung volumes and capacitie s can be measure d by a spiromet er Lung volumes and capacitie s can be measure d by a spiromet er Spirometry (In litres) findings Men Women IRV 3.3 1.9 TV 0.5 4.8 0.5 3.1 ERV 1.0 0.7 RV 1.2 1.1 TLC 6.0 4.2 Tidal volume Volume of air moved per breath  Functional Residual Capacity (FRC) helps to stabilize the composition of alveolar air - Volume of air left in the lungs after a normal, passive exhalation. - Cannot be measured by spirometry (as it includes the ERV Residual Volume [RV]) FRC RV 0 Respiratory volumes Tidal volume (TV): volume of air inhaled or exhaled in one quiet breath males = 500mls; females = 500mls (0.5L) Expiratory reserve volume (ERV): the amount of air that can be forcibly exhaled after a normal tidal volume exhalation males = 1000mls (1.0L); females = 700mls (0.7L) Inspiratory reserve volume (IRV): amount of air that can be forcibly inhaled after a normal tidal volume inhalation males = 3300mls (3.3L); females = 1900mls (1.9L) Residual volume (RV): air remaining in lungs after maximum expiration males = 1200mls (1.2L) females = 1100mls (1.1L) Respiratory Capacities Vital capacity (VC): VC = TV + IRV + ERV maximum amount of air that can be expired after a maximum inspiratory effort Males = ~4800ml, Females = ~3100ml Inspiratory capacity (IC): IC = TV + IRV maximum amount of air that can be inspired after a normal expiration Functional residual capacity (FRC): FRC = RV + ERV volume of air remaining in the lungs after a normal tidal expiration Total lung capacity (TLC): TLC = TV + IRV + ERV + RV maximum amount of air contained in lungs after a maximum inspiratory effort males = 6000mls (6.0L); females = 4200mls (4.2L)  Respiratory rates and volumes Respiratory system adapts to changing oxygen demands by varying: - number of breaths per minute ( respiratory rate) - volume of air moved per breath (tidal volume) Pulmonary Ventilation rate - also called Respiratory Minute Volume (amount of air moved per minute) = Tidal volume  Breathing frequency 500ml x 12/min = 6 litres / min Pulmonary Ventilation & Alveolar Ventilation – what is the difference? From every 500 ml Tidal Volume, 150 ml doesn’t reach alveolar exchange surfaces – Why? volume of air in conducting passages that does not participate in gas exchange = anatomical dead space This Photo by Unknown Author is licensed under CC BY-SA  Amount of air reaching alveoli each minute Respiratory rate x (TV — anatomic Alveolar dead space) ventilation 12 x (500 - 150) = 4.2 litres min-1  Larger lung Smaller lung Why might volumes volumes these measurement Male Female s be Taller people Shorter people important? Athletes Non-athletes People living at People living at high altitude low altitude Non-smokers Smokers Recap - Learning objectives of this lecture topic Part 1. Mechanics of breathing - Pressure and volume changes Part 2. What we have to overcome? Resistance to breathing: Compliance – factors affecting it including surface tension and surfactant Airway resistance: What is it What effect does it have on air flow What factors affect it How can we measure it? Part 3. Assessment of lung function Explain (and calculate where necessary) Respiratory Rates, Volumes and Capacities Describe pulmonary ventilation and the differences